Core component B will grow human xenograft tumors selected by Project 1 and conduct standard high-dose rate and low-dose-rate therapies, and measure the resulting regrowth delays. The results will be used by Project 1 to test hypotheses concerning how the time-course of irradiation interacts with characteristics of different tumor cell lines to determine tumor responses. The standard high-dose-rate therapies will be a single 10 Gy fraction or two fractionated protocols in which most radiation is given either early or late in the treatment. The standard low-doze treatments will be made with either one or two injections of 125I-labeled immunomicrospheres prepared by Project 3 to meet the treatment conditions required by Project 1. Later in the project, high- and low-dose rate treatments will be used in combination. Core B will monitor tumor retention by trace-labeling the preparations with 131I and using an external detector. Tumor dose rate distributions produced by the injections will be measured using radiochromic medium. Based on these measurements, Core B will work with project 1 to calculate how the resulting dose patterns and different models of cellular sensitivity would affect tumor response. Core B will also grow tumors and make the necessary injections for hypoxia measurements to be made by Project 2.